Rubber compositions for components for pneumatic rubber tires are usually comprised of at least one diene-based elastomer and reinforcing filler comprised of at least one of rubber reinforcing carbon black and precipitated silica. For tire treads, it is often desired for such rubber composition to promote low hysteresis for the tire component as exhibited by at least one of relative high rebound property and relative low tangent delta property for the rubber composition and also to promote abrasion resistance to thereby promote tire wear resistance.
For this invention, it is desired to evaluate providing a branched polyethylenimine oligomer (sometimes referred to as a PEI, or branched PEI oligomer) to promote at least one of such physical properties for such rubber composition, particularly for a tire with component thereof.
A branched polyethylenimine oligomer (branched PEI oligomer) is a polymer of a complex structured network comprised of in-chain amine groups with spacer between the amine groups composed of aliphatic hydrocarbon “—CH2CH2—” groups. The amine groups of branched PEI oligomers are composed of a combination of primary, secondary and tertiary amine groups. The monomer is reported as being a three-membered ring which might sometimes be referred to as an aziridine. The aziridine may be represented as:
Two corners of the monomer are composed of —CH2— linkages. The third corner is a secondary amine group, ═NH. In the presence of a catalyst, the monomer may be converted, by ring opening polymerization, into a highly branched polymer which may contain reportedly, for example, about 33 percent primary amine groups, about 40 percent secondary amine groups and about 27 percent tertiary amine group.
It is to be appreciated that the branched PEI oligomer can promote sulfur vulcanization of diene-based elastomers in a sense of being in the nature of a secondary sulfur vulcanization accelerator. An inclusion of the branched PEI oligomer in a diene-based rubber composition might be expected to promote an unwanted premature vulcanization of the rubber composition which is sometimes referred to as “scorching”. Therefore, it may be appropriate to use only a minimal amount of the branched PEI oligomer and to include a compound in the rubber composition which may act as a vulcanization retarder. However, conventional sulfur vulcanization retarder compounds such as, for example, n-cyclohexylthiophthalimide, are believed to be relatively insufficient for promoting a reduction of premature scorching when applying a branched PEI oligomer to a rubber composition. In contrast, use of carboxylic acids containing phenolic moieties such as, for example p- or m-hydroxy benzoic acid as well as salicylic acid and methylated salicylic acid, particularly salicylic acid, is considered to be more appropriate for use as a sulfur vulcanization retarder to retard premature scorching for use of a branched PEI oligomer with the diene-based elastomer-containing rubber composition.
In the description of this invention, the term “phr” is used to designate parts by weight of a material per 100 parts by weight of elastomer. The terms “rubber” and “elastomer” may be used interchangeably unless otherwise indicated. The terms “vulcanized” and “cured” may be used interchangeably, as well as “unvulcanized” or “uncured”, unless otherwise indicated.